Conventionally, there has been a compressor unit that has been used in a refrigerator having refrigerant circuits. The compressor unit has a compressor, an inverter for driving the compressor, and an over-current protective device for protecting the inverter against an output over-current. When the compressor is started, an inverter output voltage is set according to a working current value of the over-current protective device. That is, the inverter output voltage is set so that an inverter output current does not exceed the working current value of the over-current protective device and so that a maximum starting torque is gained. The over-current protective device, however, has characteristics in which a high ambient temperature decreases the working current value and in which a low ambient temperature increases the working current value, as shown in FIG. 6. When the compressor is started at a low ambient temperature, accordingly, a problem is caused in that the inverter output voltage cannot be increased though there is room for increase in the inverter output voltage resulting in increase in starting torque. When the compressor is started at a low temperature, in particular, a load is increased by increase in viscosity of oil in the compressor, accumulation of liquid refrigerant or the like. In such a case, therefore, the larger the driving torque is, the better.